Exhaust gas-driven turbochargers have a rotor which rotates in operation at a very high speed of frequently over 100,000 revolutions per minute and therefore has to be very accurately balanced for the avoidance of noises and excessive bearing loads. As a rule the rotor consists of a shaft supported in an associated bearing housing, said shaft carrying at one end a turbine wheel and at the other end a compressor wheel. For reasons of accuracy the unbalance of the rotor is measured at an angular velocity corresponding substantially to normal working speed, the rotor being rigidly installed with the bearing housing in a turbine housing and being brought to the necessary speed by applying compressed air to the turbine wheel. This method has the disadvantage that the relatively large mass comprising the bearing housing and turbine housing compared with the mass of the rotor greatly reduces the vibrations induced by the unbalance of the rotor and therefore the sensitivity and accuracy of the measurement.
From EP 0 426 676 B1 a dynamic balancing method for the high-speed rotor of an exhaust gas-driven turbocharger is known wherein the turbocharger centre housing in which the rotor is supported is rigidly connected to the housing portions enclosing the turbine wheel and compressor wheel and the unit formed thereby is soft-mounted in a framework to be movable in all three spatial dimensions with the aid of flexible bellows conduits that engage with the housing portions. Here too, the housing portions attached to the turbocharger centre housing substantially detract from the vibration characteristics and therefore from the measurement of the unbalance. On the housing portions acceleration sensors are arranged whose acceleration signals are processed in conjunction with an optically measured phase angle for determining the size and position of the unbalance.